1. Field of the Invention
The present invention relates to a driving circuit for a so-called vibration motor (vibration driven motor) for frictionally driving a moving member contacting a vibrating member by a vibration generated in the vibrating member.
2. Related Background Art
FIG. 5 shows a conventional vibration driven motor driving circuit. The circuit shown in FIG. 5 includes a microcomputer 51 for controlling a driving operation of a vibration driven motor, a D/A converter 52 for converting a digital signal output from the microcomputer 51 into an analog signal, a variable frequency oscillator (VCO) 53 whose oscillation frequency is controlled by an output voltage from the D/A converter, a D-flip-flop (D-F/F) 54 for frequency-dividing the output signal from the VCO 53 with 2, D-F/Fs 55 and 56 for generating signals, having different phases and the same frequency, for driving the vibration driven motor, an EXCLUSIVE-OR (EX-OR) gate 57 for changing the rotational direction of the vibration driven motor, AND gates 58 and 59 for controlling driving and stopping operations of the vibration driven motor, a high-voltage power source 60 for driving the vibration driven motor, power amplifiers 61 and 62 for driving the vibration driven motor, matching coils 63 and 64 for driving the vibration driven motor, and a main body 65 of the vibration driven motor.
An operation will be described below with reference to the above-mentioned circuit. The relationship between the driving frequency and the rotation speed of the vibration driven motor has characteristics, as shown in FIG. 4. In general, the driving frequency is scanned from f4 toward a lower frequency, and the frequency scan operation is stopped when the rotation speed reaches a predetermined value within a range between N1 and N2. In addition, the vibration driven motor has the following characteristics. That is, when the frequency is scanned to f0, the rotation speed becomes maximum. However, when the frequency is decreased from f0 even slightly, the rotation speed abruptly decreases.
In order to rotate such a vibration driven motor, the microcomputer 51 outputs data 00h to output ports PD0 to PD7. At this time, the output from the D/A converter becomes minimum, as shown in FIG. 6. The output from the D/A converter is input to the VCO 53, and the VCO 53 oscillates at a frequency four times the driving frequency f3, as shown in FIG. 7. The oscillation signal is input to a 90.degree. phase shift circuit constituted by the D-F/Fs 54, 55, and 56, and the phase shift circuit generates vibration driven motor driving signals having the same frequency f4 and a 90.degree. phase difference therebetween, as shown in FIG. 8.
Of these two signals, the 90.degree. phase delayed output signal from the D-F/F 56 is input to the EX-OR gate 57. The EX-OR gate 57 also receives a rotational direction signal output from the DIR terminal of the microcomputer 51. When the DIR output is at Low level, the EX-OR gate 57 outputs a 90.degree. phase delayed signal; when the DIR output is at High level, it outputs a 90.degree. phase advanced signal, thereby switching the driving direction of the vibration driven motor.
The vibration driven motor driving signals generated in this manner are respectively input to the AND gates 58 and 59. When the ON terminal of the microcomputer 51 is at High level, the AND gates 58 and 59 output signals for driving the vibration driven motor. The output signals are input to the power amplifiers using the high-voltage power source 60 as a power source, and are amplified to electric power necessary for driving the vibration driven motor. The amplified signals are respectively applied to piezoelectric elements 65a and 65b of the vibration driven motor 65 through the matching coils 63 and 64. In this case, since the applied frequency is f4, the rotation speed of the vibration driven motor becomes 0, as shown in FIG. 4, and the motor is not rotated.
The microcomputer 51 changes outputs from the ports PD0 to PD7 from 00h to 01h, so that the output voltage from the D/A converter 52 is slightly increased, as shown in FIG. 6, and the oscillation frequency of the VCO 53 is slightly decreased. For this reason, since the frequency becomes f3 (FIG. 4), the vibration driven motor 65 begins to rotate at a rotation speed N3, as shown in FIG. 4. The microcomputer 51 changes the output data from the ports PD0 to PD7 so as to increase them in turn, thereby changing the oscillation frequency of the VCO in a decreasing direction to decrease.
Thus, the driving frequency applied to the vibration driven motor 65 is scanned to gradually decrease, and the rotation speed of the vibration driven motor 65 is increased, as shown in FIG. 4. When the rotation speed of the vibration driven motor 65 reaches a target rotation speed N1, the microcomputer 51 stops scanning data to be output from the ports PD0 to PD7, and outputs constant data. The driving frequency of the signals to be applied to the vibration driven motor at this time becomes f1.
Thereafter, the microcomputer 51 causes a rotation speed detection means (not shown) to detect the rotation speed of the vibration driven motor 65 so as to maintain the constant rotation speed of the vibration driven motor 65, and changes the output data from the ports PD0 to PD7, so that the rotation speed of the vibration driven motor 65 falls within a target speed range between N1 and N2, thereby changing the output voltage from the D/A converter 52 and the oscillation frequency of the VCO. Thus, the driving frequency of the signals to be applied to the vibration driven motor is controlled, so that the rotation speed of the vibration driven motor 65 falls within a target speed range.
However, the conventional circuit requires analog circuits such as a D/A converter, a VCO, and the like. For this reason, a high-precision D/A converter, and adjustment of the oscillation frequency of a VCO are required. In addition, since analog signals are weak against noise, the analog circuits are easily influenced by a change in atmospheric temperature. In order to construct these analog circuits in an IC, digital circuits and analog circuits must be constructed using complicated processes such as a Bi-CMOS process, a linear CMOS process, and the like, resulting in a complicated and expensive circuit.